Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.02.2025 17:04, Roger Pau Monné wrote:
> On Wed, Feb 26, 2025 at 03:36:33PM +0100, Jan Beulich wrote:
>> On 26.02.2025 15:28, Roger Pau Monné wrote:
>>> On Wed, Feb 26, 2025 at 03:08:33PM +0100, Jan Beulich wrote:
>>>> On 26.02.2025 14:56, Roger Pau Monné wrote:
>>>>> On Mon, Feb 24, 2025 at 01:27:24PM +0000, Alejandro Vallejo wrote:
>>>>>> --- a/xen/common/page_alloc.c
>>>>>> +++ b/xen/common/page_alloc.c
>>>>>> @@ -490,13 +490,11 @@ static long outstanding_claims; /* total 
>>>>>> outstanding claims by all domains */
>>>>>>  
>>>>>>  unsigned long domain_adjust_tot_pages(struct domain *d, long pages)
>>>>>>  {
>>>>>> -    long dom_before, dom_after, dom_claimed, sys_before, sys_after;
>>>>>> -
>>>>>>      ASSERT(rspin_is_locked(&d->page_alloc_lock));
>>>>>>      d->tot_pages += pages;
>>>>>>  
>>>>>>      /*
>>>>>> -     * can test d->claimed_pages race-free because it can only change
>>>>>> +     * can test d->outstanding_pages race-free because it can only 
>>>>>> change
>>>>>>       * if d->page_alloc_lock and heap_lock are both held, see also
>>>>>>       * domain_set_outstanding_pages below
>>>>>>       */
>>>>>> @@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct 
>>>>>> domain *d, long pages)
>>>>>>          goto out;
>>>>>
>>>>> I think you can probably short-circuit the logic below if pages == 0?
>>>>> (and avoid taking the heap_lock)
>>>>
>>>> Are there callers passing in 0?
>>>
>>> Not sure, but if there are no callers expected we might add an ASSERT
>>> to that effect then.
>>>
>>>>>>      spin_lock(&heap_lock);
>>>>>> -    /* adjust domain outstanding pages; may not go negative */
>>>>>> -    dom_before = d->outstanding_pages;
>>>>>> -    dom_after = dom_before - pages;
>>>>>> -    BUG_ON(dom_before < 0);
>>>>>> -    dom_claimed = dom_after < 0 ? 0 : dom_after;
>>>>>> -    d->outstanding_pages = dom_claimed;
>>>>>> -    /* flag accounting bug if system outstanding_claims would go 
>>>>>> negative */
>>>>>> -    sys_before = outstanding_claims;
>>>>>> -    sys_after = sys_before - (dom_before - dom_claimed);
>>>>>> -    BUG_ON(sys_after < 0);
>>>>>> -    outstanding_claims = sys_after;
>>>>>> +    BUG_ON(outstanding_claims < d->outstanding_pages);
>>>>>> +    if ( pages > 0 && d->outstanding_pages < pages )
>>>>>> +    {
>>>>>> +        /* `pages` exceeds the domain's outstanding count. Zero it out. 
>>>>>> */
>>>>>> +        outstanding_claims -= d->outstanding_pages;
>>>>>> +        d->outstanding_pages = 0;
>>>>>> +    } else {
>>>>>> +        outstanding_claims -= pages;
>>>>>> +        d->outstanding_pages -= pages;
>>>>>
>>>>> I wonder if it's intentional for a pages < 0 value to modify
>>>>> outstanding_claims and d->outstanding_pages, I think those values
>>>>> should only be set from domain_set_outstanding_pages().
>>>>> domain_adjust_tot_pages() should only decrease the value, but never
>>>>> increase either outstanding_claims or d->outstanding_pages.
>>>>>
>>>>> At best the behavior is inconsistent, because once
>>>>> d->outstanding_pages reaches 0 there will be no further modification
>>>>> from domain_adjust_tot_pages().
>>>>
>>>> Right, at that point the claim has run out. While freeing pages with an
>>>> active claim means that the claim gets bigger (which naturally needs
>>>> reflecting in the global).
>>>
>>> domain_adjust_tot_pages() is not exclusively called when freeing
>>> pages, see steal_page() for example.
>>
>> Or also when pages were allocated. steal_page() ...
>>
>>> When called from steal_page() it's wrong to increase the claim, as
>>> it assumes that the page removed from d->tot_pages is freed, but
>>> that's not the case.  The domain might end up in a situation where
>>> the claim is bigger than the available amount of memory.
>>
>> ... is a case that likely wasn't considered when the feature was added.
>>
>> I never really liked this; I'd be quite happy to see it ripped out, as
>> long as we'd be reasonably certain it isn't in active use by people.
> 
> What do you mean with 'it' in the above sentence, the whole claim
> stuff?

Yes.

>  Or just getting rid of allowing the claim to increase as a
> result of pages being removed from a domain?

No.

Jan